[Teams] Fix quantity type conversion

[jankeromnes]

Fixes #3865

Explanation:

• element.value is always a string in HTML
• When we select a non-default value from the Team Plan Seat quantity selector, it changes quantity to a string in the React state
• Somewhere in checkout, something like newQuantity = originalQuantity + extraQuantity happens -- when extraQuantity is a string, it coerces originalQuantity to a string, and then just appends the extra digits

[jankeromnes]

Waiting on https://werft.gitpod-io-dev.com/job/gitpod-build-jx-fix-teams.0 to validate the fix

[jankeromnes]

How to test:

1. In https://jx-fix-teams.staging.gitpod-io-dev.com/teams create a new Team Plan with 10 Members
2. Go through billing (tip: pick the proposed fake credit card)
3. Once the "payment" is through, add another 10 Members to the Team Plan
4. Once the second "payment" is through, it should show 20 Members
5. When you click on Billing, it should also show Team ... x 20, not Team ... x 1010(!)

[AlexTugarev]

LGTM! Thanks for the catch!

[jankeromnes]

Thanks! Found a second, similar occurrence of setQuantity (when you create a team). Probably benign but fixing as well.

[gtsiolis]

Thanks for fixing this @jankeromnes! 💯